Just add water: when should liquefaction be considered in land use planning? GNS Science Miscellaneous Series 47

Transcription

1 Just add water: when should liquefaction be considered in land use planning? W. S.A. Saunders and K.R. Berryman GNS Science Miscellaneous Series 47 December 2012

2 BIBLIOGRAPHIC REFERENCE Saunders, W. S. A.; Berryman, K. R Just add water: when should liquefaction be considered in land use planning? GNS Science Miscellaneous Series p. W. S. A. Saunders, GNS Science, PO Box 30368, Lower Hutt K. R. Berryman, GNS Science, PO Box 30368, Lower Hutt Institute of Geological and Nuclear Sciences Limited, 2012 ISSN ISBN Cover Photo: Liquefaction in Bexley, photo credit New Zealand Herald

3 CONTENTS ABSTRACT... II KEYWORDS... II 1.0 INTRODUCTION WHAT IS LIQUEFACTION? WHAT IS PEAK GROUND ACCELERATION? WHEN SHOULD LIQUEFACTION BE CONSIDERED IN LAND USE PLANNING? What is the land use? Are the soils susceptible to liquefaction? Are the consequences of liquefaction significant? What is the likelihood that an earthquake can generate 0.1g PGA or greater at the site? Potential planning actions Greenfield sites Areas of existing development/individual land use Risk-based land use planning FUTURE GUIDANCE ACKNOWLEDGEMENTS REFERENCES FIGURES Figure 2.1 Liquefaction and its effects (IPENZ) Figure 3.1 Approximate relationship between MMI and PGA (Mercalli XII) Figure 4.1 Decision tree to assist in assessing if liquefaction should be included in land use planning Figure 4.2 An example of how consequences can be assessed (modified from W. S. A. Saunders & J. G. Beban, 2012) Figure 4.3 Relationship between PGA and Moment Magnitude for liquefaction in highly susceptible soils, Christchurch, New Zealand (Quigley, Bastin, & Bradley, 2013 (in press)) Figure National seismic hazard model for New Zealand showing expected PGA's for a 475 and 2,500 year return period earthquake (10% and 2% exceedence in 50 years respectively) for shallow soils (class C) (Stirling et al., 2012, p1531) GNS Science Miscellaneous Series 47 i

4 ABSTRACT Many local authorities are currently investigating liquefaction hazard and exploring land use planning methods to mitigate potential risk. While in many cases the need to investigate liquefaction hazard is required, anecdotal evidence indicates that several councils are unduly concerned with liquefaction risk. This is because liquefaction requires the following basic conditions in order to occur: an earthquake large enough to trigger liquefaction within planning timeframes; specific soil characteristics; and a high water table. The purpose of this report is to provide guidance to land use planners and decision makers so they can assess whether liquefaction is a hazard that needs to be included in the planning process. To achieve this, the report provides an explanation of liquefaction and earthquake thresholds (quantified by peak ground acceleration), followed by a simple decision tree designed for planners to ensure that liquefaction is appropriately included in land use planning decisions. Each of the steps in the decision tree is discussed in further detail. Key questions include: Are the soils susceptible to liquefaction? Are the consequences of liquefaction significant? What is the likelihood of an earthquake generating ground motions at a site above a liquefaction threshold of 0.1g peak ground acceleration occurring? Concluding the report is an overview of future research into liquefaction and its management. This report does not provide guidance on how to include liquefaction into planning documents additional multi-disciplinary guidance to assist with this is being developed through other research and policy (e.g. Brackley et al., 2012; MBIE, 2012; NZGS, 2010). KEYWORDS Liquefaction, land use planning, peak ground acceleration (PGA), earthquake likelihood, consequences, risk-based planning. GNS Science Miscellaneous Series 47 ii

5 1.0 INTRODUCTION The 2010 and 2011 Canterbury earthquakes have illustrated the damage to infrastructure, the disruption to economic activity and the social consequences (displacement of individuals and communities) that can result from liquefaction. The potential for liquefaction is not limited to Christchurch and surrounds, with several cities and towns in New Zealand built on potentially liquefiable land. Liquefaction occurrence in Canterbury is not the first occurrence in New Zealand; liquefaction has been documented in association with almost all moderate to large earthquakes in New Zealand (e.g. Berrill, Mulqueen, & Ooi, 1994; Carr & Berrill, 2004; Christensen & Berrill, 1994; Fairless & Berrill, 1984; Hancox, 2005; Hancox, Perrin, & Dellow, 2002). Many local authorities are currently investigating liquefaction hazard and exploring land use planning methods to address potential risk. While in many cases the need to investigate the liquefaction hazard is required, anecdotal evidence indicates that several councils may be unduly concerned with liquefaction risk. This is because liquefaction requires three coincident factors: specific soil characteristics; a high water table; and earthquakes large enough to trigger liquefaction within land use planning timeframes. If an area does not meet one or more of these basic conditions, there is no need to consider the liquefaction hazard as part of land use planning process. The purpose of this report is to allow planners to assess if liquefaction is a hazard that should be included in the planning process. To achieve this, the report provides an explanation of liquefaction and earthquake shaking minimum thresholds, described in terms of peak ground acceleration (PGA). This is followed by a simple decision tree to assist planners to ensure that liquefaction is appropriately incorporated into land use plans and considered in resource consent applications. Following this we provide an explanation of each of the key parts to the decision tree. The first four are questions to assist in evaluating whether liquefaction should be included in land use planning, and the final part provides some initial suggestions for how liquefaction could be incorporated into land use planning. This report does not provide guidance on how to include liquefaction into planning documents additional multidisciplinary guidance to assist with this is being developed through other research and policy (Brackley, et al., 2012; MBIE, 2012; NZGS, 2010). GNS Science Miscellaneous Series 47 1

6 2.0 WHAT IS LIQUEFACTION? This section is adapted from the Institution of Professional Engineers of New Zealand Liquefaction fact sheet (IPENZ). Liquefaction is the process that leads to a soil suddenly losing strength, most commonly as a result of ground shaking during a large earthquake. Not all soils however, will liquefy in an earthquake. The following are particular features of soils that potentially can liquefy: Loose sands and silts. Such soils do not stick together the way clay soils do. They are below the water table, so all the space between the grains of sand and silt is filled with water. Dry soils above the water table will not liquefy. When an earthquake occurs, the shaking may be so strong that the sand and silt grains try to compress the spaces filled with water, but the water pushes back and pressure builds up until the grains float in the water. Once that happens the soil loses its strength it has liquefied. Soil that was once solid now behaves like a fluid. Liquefied soil, like water, cannot support the weight of whatever is lying above it be it the surface layers of dry soil, the concrete floors, or piles of buildings. The liquefied soil under that weight is forced into any cracks and crevasses it can find, including those in the dry soil above, or the cracks between concrete slabs. It flows out onto the surface as boils, sand volcanoes and rivers of silt. In some cases the liquefied soil flowing up a crack can erode and widen the crack to a size big enough to accommodate a car. Some other consequences of the soil liquefying are: Differential settlement of the ground surface due to the loss of soil from underground; Loss of support to building foundations; Floating of manholes, buried tanks and pipes in the liquefied soil - but only if the tanks and pipes are mostly empty; and Near streams and rivers, the dry surface soil layers can slide sideways on the liquefied soil towards the streams. This is called lateral spreading and can severely damage a building. It typically results in long tears and rips in the ground surface. Liquefaction may not affect all of the foundations of a building. The affected part may subside (settle) or be pulled sideways by lateral spreading, which can severely damage the building. Buried services such as sewer pipes can be damaged as they are warped by lateral spreading, ground settlement or floatation. GNS Science Miscellaneous Series 47 2

7 Figure 2.1 Liquefaction and its effects (IPENZ).

8 3.0 WHAT IS PEAK GROUND ACCELERATION? When discussing triggers for liquefaction, peak ground acceleration (PGA) is often referred to. A consequence of an earthquake, PGA is a measure of ground acceleration at a particular site by instruments. PGA is expressed in g, being the acceleration due to the earth s gravity equivalent to a g-force. Unlike the Richter (Ms) or moment magnitude (Mw) scales, PGA is not a measure of the total energy (i.e. magnitude or size) of an earthquake, but how much the earth shakes at a given place (where the recording instrument is located). Whereas the Modified Mercalli Intensity (MMI) scale uses personal reports and observations to measure earthquake intensity, and is therefore more subjective, the PGA is an instrumental measurement of ground shaking. As an indication of PGA force, an earthquake that results in 0.2g may cause people to lose their balance and is approximately equivalent to MM7 (see Figure 3.1). MMI IV PGA (g) 0.03 and below V VI VII VIII IX X XI XII 0.90 and above Figure 3.1 Approximate relationship between MMI and PGA (Mercalli XII). GNS Science Miscellaneous Series 47 4

9 4.0 WHEN SHOULD LIQUEFACTION BE CONSIDERED IN LAND USE PLANNING? A simple decision tree (Figure 4.1) is presented to assist planners deciding whether liquefaction is a hazard that needs to be considered within planning processes or not. Key aspects are outlined in further detail in the following subsections. Figure 4.1 Decision tree to assist in assessing if liquefaction should be included in land use planning. GNS Science Miscellaneous Series 47 5

10 4.1 WHAT IS THE LAND USE? In order to assess if research is required to ascertain the susceptibility to liquefaction, first the the existing or proposed future land use needs to be considered. For example, is the land used for agriculture? Residential? Commercial? Lifelines/critical facilities? School? Retirement village? Once the existing or future land use as been ascertained, then the susceptibility of the location to liquefaction (section 4.2) and the significance of the consequences (section 4.3) can be assessed. 4.2 ARE THE SOILS SUSCEPTIBLE TO LIQUEFACTION? Not all soils are susceptible to liquefaction. Generally, for liquefaction to occur there needs to be three soil preconditions: 1. Geologically young (predominantly Holocene (less than 10,000 years old), loose (i.e. not compacted) sediments, that are 2. fine-grained and non-cohesive (coarse silts and fine sands), and 3. the water table is less than 5 metres below the surface, i.e. susceptible soils are saturated. If one or more of these preconditions are not met, then soils are not susceptible to liquefaction and generally no planning is required. If all three of these preconditions are met, then an assessment of the earthquake hazards is required. It is important to note that the saturated condition may apply seasonally or only part of the time i.e. the potential for saturation must be assessed. 4.3 ARE THE CONSEQUENCES OF LIQUEFACTION SIGNIFICANT? Once it has been ascertained that soils are susceptible to liquefaction, then an assessment of the consequences of liquefaction on that land use needs be undertaken. The primary impacts of liquefaction are to the built environment (e.g. buildings: commercial, residential, etc); infrastructure (i.e. underground pipes and services, roads); and to socio-economic resilience if people are not able to live in their homes and/or attend places of education and employment. For critical facilities, damaging liquefaction should not impact on continued functionality of the facility in a 1 in 2500 year event. If the impacts of liquefaction are insignificant, then it may be appropriate that no planning actions are required. If, however, the potential consequences are significant, then liquefaction should be a criteria for land use planning. An example of how consequences can be assessed is provided in Figure 4.2. GNS Science Miscellaneous Series 47 6

11 Figure 4.2 An example of how consequences can be assessed (modified from W. S. A. Saunders & J. G. Beban, 2012). Once the consequences have been assessed, then the likelihood of an event occurring that can produce liquefaction is required. 4.4 WHAT IS THE LIKELIHOOD THAT AN EARTHQUAKE CAN GENERATE 0.1G PGA OR GREATER AT THE SITE? Not all locations in New Zealand will experience an earthquake with ground shaking that will result in damaging liquefaction within reasonable planning timeframes. As Figure 4.3 shows, for New Zealand conditions generally a PGA of 0.1g or more is enough to trigger damaging liquefaction when combined with highly susceptible soil conditions and high water table (as outlined in Section 4.2). However, this threshold can be increased or decreased depending on the local conditions and duration of the earthquake. The likelihood that liquefaction will occur (classified here as rare, unlikely, possible, likely and almost certain) depends on the combination of the susceptibility criteria, and the range of possible earthquake events. GNS Science Miscellaneous Series 47 7

12 Figure 4.3 Relationship between PGA and Moment Magnitude for liquefaction in highly susceptible soils, Christchurch, New Zealand (Quigley, Bastin, & Bradley, 2013 (in press)). The National Seismic Hazard Model for New Zealand provides an indicative assessment of expected PGA's for a 475- and 2,500- year return period earthquake (10% and 2% exceedence in 50 years respectively - see Figure 4.4) for shallow soils (class C of the Standards New Zealand :2004 Structural design actions - earthquake). Equivalent maps for deep or soft soils (classes D and E of the NZS :2004 Structural design actions - earthquake) are required to underpin liquefaction risk assessment (Standards Australia/New Zealand, 2004). GNS Science Miscellaneous Series 47 8

13 Figure National seismic hazard model for New Zealand showing expected PGA's for a 475 and 2,500 year return period earthquake (10% and 2% exceedence in 50 years respectively) for shallow soils (class C) (Stirling et al., 2012, p1531). Figure 4.2 should be used as an initial screening tool only - further scale-appropriate assessment will be required to assess actual expected PGAs for both shallow and deep soil conditions. If the soil preconditions outlined in Section 4.2 are all present and the location is likely to experience PGA of 0.1g or more during the period of interest, then there probably is a liquefaction hazard requiring more detailed evaluation and consideration in land use planning. Planning options are outlined in the following section. 4.5 POTENTIAL PLANNING ACTIONS There are opportunities to plan for liquefaction hazard and risk in both greenfield sites and in areas of existing development Greenfield sites Greenfield sites provide a unique opportunity to undertake prospective planning, i.e. take planning actions that address and seek to avoid future unacceptable impacts (UNISDR, 2009). In taking a risk-based approach to planning policies and consents, planning criteria would become more restrictive as the risk (socio-economic, built or life safety) increases. Before development occurs there is an opportunity to model future impacts and consequences of proposed developments and reconsider development options if required. A range of ground improvement techniques can also be considered to reduce risk to acceptable levels, if there is reasonable benefit for costs invested in ground engineering. GNS Science Miscellaneous Series 47 9

14 4.5.2 Areas of existing development/individual land use Areas of existing development require a combination of prospective and corrective planning (i.e. planning actions that address and seek to correct or reduce risks which are already present (UNISDR, 2009)) to ensure that future risks are not increased. Examples of how this can be achieved include: restrict further development (i.e. infill housing, new critical facilities); relocate critical facilities/infrastructure to lower risk areas; strengthen buildings, infrastructure or the soil to ensure they are resilient to future liquefaction effects and to reduce the risk to an acceptable level (i.e. as low as reasonably practicable); and promulgate district plan provisions that ensure changes of land use do not increase risks e.g. a residential dwelling becoming a child care centre. Pre-event recovery planning (e.g. Becker, Saunders, Hopkins, Wright, & Kerr, 2008) provides an opportunity to give some thought to how an area could be redeveloped during the recovery phase of a major event, and also the planner s role during recovery. For example, having planned for appropriate disposal areas for the liquefaction material, with any necessary consents for disposal granted, or planned retreat from areas of high risk. In many cases, the existing risk needs to be managed with emergency management preparedness. This includes having instructions for landowners on how to collect and dispose of material; having machinery available to remove the material; and having disposal areas already consented Risk-based land use planning A risk-based approach involves evaluating the consequences of an event, assessing the likelihood, and then having planning and/or engineered ground improvement provisions (i.e. policies, consent, and/or ground improvement requirements) relevant to the risk. For example, as risk increases, the resource consent categories become more restrictive. Further details on the risk-based approach can be found in Saunders & Beban (2011; 2012) and Saunders (2012). GNS Science Miscellaneous Series 47 10

15 5.0 FUTURE GUIDANCE A number of initiatives are underway or being planned which will provide further guidance to planners on how to address liquefaction hazard and risk. Three proposed areas of guidance are outlined below: This planning note is the preliminary step of a long-term objective to develop multidisciplinary guidance on liquefaction, which will integrate geotechnical, engineering and land use planning information requirements. This guidance will be developed once lessons have been learned from liquefaction in Christchurch (e.g. MBIE, 2012; NZGS, 2010). A three-step risk-based planning approach to natural hazards has been developed by Saunders (2012; Saunders & Beban, 2011) and is being expanded in a Ministry of Business, Innovation and Employment (previously the Ministry of Science and Innovation) Envirolink Tools project, due for release in August This risk-based approach has a focus on consequences before determining the likelihood of an event. First, the social, economic, built and health and safety consequences are described from insignificant and minor through to moderate, major and catastrophic. The likelihood of an event that would produce those consequences is then determined. Finally, risk is assessed (consequence x likelihood) and the risk-based approach to policy and resource consents can be implemented, where consents become more restrictive as the risk increases. In July 2012 a Government-appointed Technical Advisory Group (TAG) released a report and recommendations on changes to the Resource Management Act 1991 (RMA). Included was a review of the natural hazard provisions in the RMA, with recommendations on how liquefaction could be addressed in the RMA. The implications of the recommendations on natural hazards are outlined in Saunders & Beban (2012), which is available on the GNS Science website GNS Science Miscellaneous Series 47 11

16 6.0 ACKNOWLEDGEMENTS This report was co-funded by the Natural Hazards Research Platform and the Wellington It s Our Fault project. We would like to thank our reviewers, Sally Dellow from GNS Science, Martin Butler from Bay of Plenty Regional Council, and James Beban from Cuttriss Consultants. 7.0 REFERENCES Becker, J., Saunders, W. S. A., Hopkins, L., Wright, K., & Kerr, J. (2008). Pre-event recovery planning for land use in New Zealand: an updated methodology. Lower Hutt: GNS Science. Berrill, J. B., Mulqueen, P. C., & Ooi, E. T. C. (1994). Liquefaction at Kaiapo in the 1901 Cheviot, New Zealand, earthquake. Bulletin of the New Zealand National Society for Earthquake Engineering, 27(3), Brackley, H. L., Almond, P., Barrell, D. J. A., Begg, J. G., Berryman, K. R., Christensen, S., et al. (2012). Review of liquefaction hazard information in eastern Canterbury, including Christchurch City and parts of Selwyn, Waimakariri and Hurunui Districts (M). Carr, K. M., & Berrill, J. B. (2004). Liquefaction case histories from the West Coast of the South Island, New Zealand. Christchurch: Department of Civil Engineering, University of Canterbury. Christensen, S. A., & Berrill, J. B. (1994). Study of liquefaction in the 1987 Edgecumbe earthquake: Landing Road bridge. Paper presented at the New Zealand National Society for Earthquake Engineering Technical Conference & AGM. Fairless, G. J., & Berrill, J. B. (1984). Liquefaction during historic earthquakes in New Zealand. Bulletin of the New Zealand National Society for Earthquake Engineering, 17(4), Hancox, G. T. (2005). Landslides and liquefaction effects caused by the 1855 Wairarapa earthquake: then and now. Paper presented at the The 1855 Wairarapa Earthquake Symposium: 150 years of thinking about magnitude 8+ earthquakes and seismic hazard in New Zealand. Hancox, G. T., Perrin, N. D., & Dellow, G. D. (2002). Recent studies of historical earthquakeinduced landsliding, ground damage and MM intensity in New Zealand. Bulletin of the New Zealand National Society for Earthquake Engineering, 35(2), IPENZ. Liquefaction. Retrieved 18 July 2012, from MBIE. (2012). Guidelines for the investigation and assessment of subdivisions on the flat in Canterbury: minimum requirements for geotechnical assessment for land development ('flatland areas' of the Canterbury region). Wellington: Ministry of Business, Innovation and Employment. Mercalli XII. Seismic intensity scales vs peak ground acceleration. Retrieved 28 November, 2012, from GNS Science Miscellaneous Series 47 12

17 NZGS. (2010). Geotechnical earthquake engineering practice: Module 1 - Guideline for the identification, assessment and mitigation of liquefaction hazards (July 2010 ed.): New Zealand Geotechnical Society Inc. Quigley, M., Bastin, S., & Bradley, B. (2013 (in press)). Recurrent liquefaction in Christchurch, New Zealand during the Canterbury earthquake sequence. Geology. Saunders, W. S. A. (2012). Innovative land use planning for natural hazard risk reduction in New Zealand. Unpublished PhD thesis, Massey, Palmerston North. Saunders, W. S. A., & Beban, J. G. (2011). Risk-based approach to natural hazards. Planning Quarterly, 183, Saunders, W. S. A., & Beban, J. G. (2012). A framework for risk-based land use planning for natural hazards. In GNS Science (Ed.), 6th Australasian Natural Hazards Management Conference. Christchurch: GNS Science. Saunders, W. S. A., & Beban, J. S. (2012). Putting R(isk) in the RMA: Technical Advisory Group recommendations on the Resource Management Act 1991 and implications for natural hazards planning. GNS Science Miscellaneous Series 48. Standards Australia/New Zealand. (2004). Standard design actions - Earthquake actions New Zealand, AS/NZS :2004: Standards New Zealand. Stirling, M., McVerry, G., Gerstenberger, M., Litchfield, N., Van Dissen, R., Berryman, K. R., et al. (2012). National seismic hazard model for New Zealand: 2010 update. Bulletin of the Seismological Society of Amercia, 102(4), UNISDR. (2009). Terminology on disaster risk reduction: United Nations International Strategy for Disaster Reduction. GNS Science Miscellaneous Series 47 13